The only warning I get is about my scipy missing umfpack. Is that the
appropriate runtime warning?
If not a runtime warning about a missing mumps might be a good idea.
You might be right. MUMPS is not necessary for Kwant to work, and the
performance of umfpack used to be quite OK for many applications. Now that umfpack is no longer included in SciPy, without MUMPS Kwant is indeed very slow.
The current master of Kwant has a much clearer warning, which goes exactly what you're discussing, check it out and tell if you find it satisfactory. I removed the mention of umfpack: it's gone from scipy due to software rot. Instead I just complain when there's no mumps.
Also, is mumps detected at build or runtime or has the build.conf be
There is a very simple auto detection that works at least on
Debian/Ubuntu . In cases where this fails build.conf must be filled manually. If you can extend the auto detection to work on Arch as well, we will be more than happy to take this over.